why do cheap RFID125 readers RDM6300 not work?

while experimenting with arduino RFID tag emulation I came across the fact that cheap readers from > aliexpress like RDM6300 do NOT work. they read very seldom, even with normal tags, and only in specific positions or with specific motion. but some time ago I used a similar reader in a Remote Switch project and it worked OK. price for this reader was 10$ not 1.5$ as it is now. so i try experimenting how i can improve reader operation.

schematic of the input circuit is the following:

what i found:
1. after some experimenting I found that Op-amp do not like negative pulses and like more positive bias in input signal, so adding schottky diode to the clamp circuit significantly improves operation.
2. Op-amp amplification is set to 1000 (1 Mohm/1 Kohm resistors) that is relatively high. decreasing amplification for the first Op-amp is making operations more stable. So i shorted 1 Mohm resistor with 100 Kohm resistor to get amplification around 100.
3. C3 5 uF capacitor in a filter circuit has too much of a capacitance. because of this, the motion of tags creates biases for Op-amp and makes reading very motion dependent. this actually makes reading possible in the original variant because this can create needed bias to trigger op-amp. but now it's better to change it to 100 nF capacitor.

resulting schematic:

and pics:

so now two of my readers read the standard tags OK. But they are not ideal with arduino emulated tags because they have deeper modulation and the reader CPU likes positive and negative pulse in bit to be of very similar length. when a strong signal starts clipping on diodes then the symmetry of the signal tends to be shifted and so emulated tags work at specific distance ranges.

my theory is that use of subpar Op-amp and not optimal components value created this problem.

aleks3d